1. Field of the Invention
The invention relates to electric arc welding and particularly to a power supply for gas tungsten arc welding.
2. Description of the Prior Art
Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, is a well-known process in which a current flows between a non-consumable welding electrode, usually tungsten, and a workpiece, typically aluminum. GTAW operations with alternating current and direct current with the electrode negative (so-called straight polarity) or positive (so-called reverse polarity) are familiar to individuals skilled in the welding art. Similarly, welding processes using pulsed power and using supplemental radio-frequency power to provide better arc ignition or arc stabilization are well known.
Several factors influence the selection of the current wave shape for alternating current GTAW. Welding occurs primarily when the electrode is negative, but the electrode is made positive periodically to clean it. Because the electron emissivity of aluminum is much lower than that of tungsten, a higher potential must be applied to establish an arc when the electrode is positive, but too high a positive potential leads to migration of tungsten from the electrode. The positive potential should have a flat top in order to reduce overheating and possible melting of the tungsten electrode. The transition between negative and positive polarity must be accomplished quickly in order to reignite the arc and minimize rectification in the arc. Experience has shown that the optimum current wave shape is an asymmetric square wave, or rectangular wave. Preferably, both the magnitude and duration of the positive and negative current portions are adjustable to optimize the workpiece heating, electrode cleaning and other parameters for a particular welding situation.
Magnetic amplifiers or saturable reactors have been used for inexpensive power supplies for GTAW. Two independent direct current supplies with an associated switching network are more expensive but provide better control of wave shape. Such power supplies are described in U.S. Pat. Nos. 3,330,933 to Maklary, 3,999,034 to Barhorst, and 3,904,846 to Risberg (FIG. 2E). U.S. Pat. Nos. 3,382,345 to Normando, 3,598,954 to Iceland, 3,894,210 to Smith et al, 3,904,846 to Risberg (FIG. 2B), and 4,092,517 to Woodacre disclose welding power supplies having a single direct current source coupled with an inverter to provide an alternating current output. Some of the power circuits of the preceding Risberg and Normando patents employ a center-tapped inductor to commutate silicon controlled rectifiers. The circuit of U.S. Pat. No. 4,038,515, also to Risberg, combines an input transformer, a single inductor and a bridge of four silicon controlled rectifiers which are fired at selected times synchronized with the input potential. In most of the circuits in the two patents to Risberg, a controlled rectifier provides a free wheeling path for inductor current during a portion of the cycle.